Summary

In the next 100 years, it is projected that Earth will move to a greenhouse climate state (1). The future ocean will not only be hotter but also more acidic and will contain extended zones with reduced oxygen (2, 3). Study of past periods of global warming helps to project what Earth and its biota will be like in this new state and what the journey to that state will entail. On page 366 in this issue, Sun et al. (4) show that beginning with the end-Permian mass extinction (∼252.6 million years ago) and continuing for the next 5 million years, Earth's oceans were extremely hot, with stressful and commonly lethal effects on ocean life.